1. Field of the Invention
This invention broadly relates to filtering methods and apparatus. More particularly, it concerns the removal of solids of small size from aqueous suspensions, e.g., waste water treatment effluent, by methods that can remove the undesired solids with an effectiveness at least as good as the prior art while reducing up to 80% the wash water normally required for the wash cycle and apparatus to perform such methods.
2. Description of the Prior Art
The efficient removal of low concentration, suspended solids (LCSS) present at relatively low concentration in aqueous suspension to produce an effluent meeting quality standards is a continuing problem in waste water treatment. Many other liquid treatment operations, e.g., potable water production involving removal of solids present in surface waters pretreated by coagulation and sedimentation, also present design engineers with the same problem.
Granular media filtration (GMF) is an established method used in solids removal from potable water (see W. J. Weber, Jr., Physiochemical Processes for Water Quality Control, Wiley Interscience, N.Y. N.Y., 1972). GMF is also used in advance waste water treatment for removal, inter alia, of residual biological floc in settled effluents from secondary treatment, so called "tertiary filtration". Basically, such operations involve the use of six different configurations for the granular media filters, namely, (a) conventional sand bed, (b) dual media, (c) triple (mixed) media, (d) single media unstratified, (e) upflow or immedium and (f) bi-flow (see EPA publication 625/4-74-007a on Wastewater Filtration--Design Considerations--Rev. 1977, pps. 8-11). In addition to such established filtration methods, there are various proprietary filter methods and equipment used for LCSS removal (see EPA publication 625/1-75-003a on Process Design Manual for Suspended Solids Removal--1975 and U.S. Pat. No. Re. 28,458; U.S. Pat. No. 3,587,861 and 3,840,117.)
As is well known by design engineers and others concerned with LCSS removal from aqueous suspensions, a variety of difficulties serve to reduce the efficiency and increase the costs of such operations. For example, as filtration progresses, suspended solids collect on the surface and/or interior of the filtration unit causing headloss development. When the headloss exceeds desirable operating level, removal of filtered solids by backwashing or other means becomes necessary to restore the effectiveness of the filter system. In the established types of CMF mentioned above, relatively large amounts of wash water are required. Since the wash water contains the suspended solids removed from the filter media, it is generally recycled in the system to capture such solids. This hydraulic loading of the aqueous treatment system reduces its overall capacity and increases costs of operation. Special wash water handling systems have been developed in an attempt to mitigate this problem (see U.S. Pat. No. 3,792,773).
Another problem with GMF arises when backwashing is not properly controlled since serious damage can occur to the filter bed, e.g., "blowing" of the bed or intermixing of media. Even if such damage does not result, backwashing time is unproductive of quality effluent and the longer it is, the lower is the overall efficiency of the filtering operation.
Even though the art of LCSS removal from aqueous suspensions by filtration has been highly developed through many years of usage coupled with continued design changes, there continues to exist a need for further improvements in such methods and apparatus to reduce equipment, operating and maintenance costs, reduce wash water volume and time requirements and improve overall efficiency.
In the proprietary filter methods developed heretofore, it has been known to use rigid filter modules (RFM), i.e., rigid plates, sheets, or the like formed of particulate aggregate fixed in a matrix of inorganic ceramic or organic resin (see U.S. Pat. No. 1,576,440). While such RFM have been used in the prior art for special purposes, e.g., filtering hydrofluoric acid and other acid solutions, they have not been utilized in LCSS filtration. The present invention provides RFMs of unique construction and provides new methods for LCSS removal from aqueous suspensions using such new filter media.